1. Field of the Invention
The principles of the present invention relate generally to the field of optical network analysis, and more particularly, to a system and method for determining the optical characteristics of an optical component under test.
2. Description of Related Art
An optical network analyzer is a vital tool for determining optical characteristics of optical components, such as fiber Bragg gratings. The optical characteristics determined by an optical network analyzer may include reflectance and transmittance of a particular two port or multiport optical component under test. The optical characteristics (e.g., reflection or transmission) of an optical component under test are described by a transfer function and are typically determined by measuring the amplitude and phase of optical signals that have been reflected by or transmitted through the component. The phase response characteristics of an optical component under test are often described by group delay or dispersion.
Most conventional group delay measurement techniques were developed for optical network analyzers that utilize non-continuously tunable laser sources. A non-continuously tunable laser source is a laser source that can be tuned across a predefined range of frequencies in discrete frequency steps. A typical technique to measure amplitude, phase, and group delay is to apply an electrical stimulus in the form of intensity or phase modulation to an optical signal, and then measure the electrical response of the transmitted or reflected optical signals using a well known phase sensitive electrical detection device, e.g., a lock-in amplifier or an electrical network analyzer. A concern with this technique is that the group delay measurement is indirect and limits the accuracy of the measurement. In addition, the technique requires a long measurement time to obtain an accurate result. Consequently, a long-term stability of the test setup is required to effectively utilize the technique.
However, continuously tunable laser sources have recently become available. A continuously tunable laser source can continuously sweep a predefined range of frequencies without frequency jumps or mode hops. The availability of continuously tunable laser sources has allowed for development of interferometric methods for measuring the optical characteristics of optical components. The interferometric methods are based on direct measurements of phase differences between interfering optical signals. Typically, Fourier analysis of a heterodyne beat frequency directly related to the sweeping optical frequency of a continuously tunable laser source is used to measure the optical characteristics, including the group delay. A concern with the interferometric methods using Fourier analysis is that the frequency sweep of a continuously tunable laser source is non-uniform. The non-uniformity of the frequency sweep causes a similar non-uniformity in the resulting beat frequency that introduces an uncertainty in the calculation of the optical characteristics by means of the Fourier analysis. What is needed is a technique to determine the optical characteristics of the continuously tunable laser with minimal uncertainty.